Version independence for object oriented programs

ABSTRACT

A method, system and program for isolating the executable binary form of computer applications that use object definition libraries from changes in the implementation or specification of object definitions in the library. These objects include adding new methods to an object definition; moving the point of definition for one of the class methods to the class parent class; changing private instance data associated with an object definition; inserting a new parent class definition between the class and its parent class when it has one; and changing the implementation of one of the class methods without changing the methods interface. The objects are achieved by removing offset and size values from the application binary image and putting them in data structures that are initialized at runtime.

This is a continuation of application Ser. No. 07/805,663 filed Dec. 12,1991, now abandoned.

FIELD OF THE INVENTION

This invention generally relates to improvements in object orientedprograms and more particularly to solving problems arising from theindependent evolution of object definition libraries and theapplications that use them.

BACKGROUND OF THE INVENTION

Among developers of workstation software, object-oriented programming(or OOP) is increasingly recognized as an important new programmingtechnology. It offers expanded opportunities for software reuse andextensibility, with improved programmer productivity when compared toconventional software development paradigms. Even so, object-orientedtechnology has not effectively penetrated major commercial softwareproducts to date. In particular, operating-systems have hesitated toembrace the new technology.

As with many new programming technologies, the early expressions of OOPconcepts focused on the creation of new languages and toolkits, eachdesigned to exploit some particular aspect. So-called pureobject-oriented languages, such as Smalltalk, presume a completerun-time environment (sometimes known as a virtual machine) becausetheir semantics represent a major departure from traditionalprocedurally oriented system architectures. Hybrid languages such asC++, on the other hand, require less run-time support but sometimesresult in tight bindings between programs that provide objects and theclient programs that use them. Tight binding between object-providingprograms and their clients often require client programs to berecompiled whenever simple changes are made in the providing programs.Examples of such systems are found in U.S. Pat. Nos. 6,885,717;4,953,080 and 4,989,132.

Because different languages and object-oriented toolkits emphasizedifferent aspects of OOP, the utility of the resulting software isfrequently limited in scope. A C++ programmer, for example, cannoteasily use objects developed in Smalltalk, nor can a Smalltalkprogrammer make effective use of C++ objects. Objects and classesimplemented in one language simply cannot be readily used from another.Unfortunately when this occurs one of the major benefits of OOP, theincreased reuse of code, is severely curtailed. Object-oriented languageand toolkit boundaries become, in effect, barriers to interoperability.

SUMMARY OF THE INVENTION

Accordingly, it is a primary objective of the present invention toisolate the executable binary form of computer applications that useobject definition libraries from changes in the implementation orspecification of object definitions.

The following changes can be made to an object definition withoutcompromising its use by the unmodified executable binary form of acomputer application which dynamically loads the object definition eachtime the application is executed:

new methods can be added to an object definition;

move the point of definition for one of the class methods to the classparent class;

add, delete, op otherwise change private instance data associated withan object definition;

insert a new parent class definition between the class and its parentclass when it has one; and

change the implementation of one of the class methods without changingthe methods interface.

These and other objectives of the present invention ape accomplished bythe operation of an algorithm in the memory of a processor employing thefollowing techniques. Removal of method and instance data offsets froman application employing the following steps. In static object models,an offset is used to select the method procedure for each particularmethod name from a method procedure table. This offset depends on thenumber and order of the methods of the class, which class the method isdefined in, and the number of methods defined by the class' ancestors.

In System Object Model (SOM) Processing, the offsets associated withmethods ape collected into a single memory data structure for eachclass, called the class data structure. This data structure is given anexternal name and its contents ape referred to in application binaryimages. Each class data structure is initialized to contain theappropriate offset values when the class object is initialized. Thus,each time an application is executed, all of the offset values arerecalculated based on the current definitions of the classes used by theapplication.

A similar problem arises with public instance data. An application thataccesses a public instance variable contained in an application object'sstate data structure must employ an offset into the object's state datastructure. This problem is solved by putting the offset for each publicdata variable in the class data structure. Each class data structure isinitialized to contain the appropriate offset values when a class objectis initialized. Thus, each time an application is executed all of theoffset values are recalculated based on the current definitions of theclasses used by the application.

When new instances of objects are created, a correct amount of computermemory must be allocated to hold the object's state data structure. InSOM this value is available via a call to the object's class object andtherefore need not be contained in an application's binary image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a personal computer system in accordancewith the subject invention;

FIG. 2 is a drawing of a SOM data structure in accordance with thesubject invention;

FIG. 3 is a drawing of a SOM class data structure in accordance with thesubject invention;

FIG. 4 is a flowchart depicting a language neutral object interface inaccordance with the subject invention;

FIG. 5 is a flowchart depicting a link, load and execution of anapplication using SOM objects in accordance with the subject invention;

FIG. 6 is a flowchart depicting the creation of a new SOM class inaccordance with the subject invention;

FIG. 7 is a flowchart depicting the detailed construction of a new SOMclass in accordance with the subject invention;

FIG. 8 is a flowchart depicting the detailed construction of a new SOMgeneric class object in accordance with the subject invention;

FIG. 9 is a flowchart depicting the detailed initialization of a new SOMclass object in accordance with the subject invention;

FIG. 10 is a flowchart depicting the detailed initialization of a SOMclass data structure with offset values in accordance with the subjectinvention;

FIG. 11 is a flowchart depicting the detailed parent class shadowing ofa statically defined class hierarchies in accordance with the subjectinvention;

FIG. 12 is a flow diagram depicting the redispatch method in accordancewith the subject invention;

FIG. 13 is a flowchart depicting the detailed initialization of theoffset value in a SOM class data structure for a single public instancevariable in accordance with the subject invention;

FIG. 14 is a flowchart depicting the detailed control flow that occurswhen a redispatch stub is employed to convert a static method call intoa dynamic method call in accordance with the subject invention; and

FIG. 15 is a flowchart depicting the detailed control flow thatinitialize a method procedure table for a class in accordance with thesubject invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is preferably practiced in the context of an operatingsystem resident on an IBM PS/2 computer available from IBM Corporation.A representative hardware environment is depicted in FIG. 1, whichillustrates a typical hardware configuration of a workstation inaccordance with the subject invention having a central processing unit10, such as a conventional microprocessor, and a number of other unitsinterconnected via a system bus 12. The workstation shown in FIG. 1includes a Random Access Memory (RAH) 16, Read Only Memory (ROM) 16, anI/O adapter 18 for connecting peripheral devices such as disk units 20to the bus, a user interface adapter 22 for connecting a keyboard 26, amouse 26, a speaker 28, a microphone 32, and/or other user interfacedevices such as a touch screen device (not shown) to the bus, acommunication adapter 36 for connecting the workstation to a dataprocessing network and a display adapter 36 for connecting the bus to adisplay device 38. The workstation has resident thereon the OS/2 baseoperating system and the computer software making up this inventionwhich is included as a toolkit.

Object-Oriented Programming is quickly establishing itself as animportant methodology in developing high quality, reusable code. Theinvention includes a new system for developing class libraries andObject-Oriented programs. This system is called the System Object Model(SOM). A detailed description of object oriented programming, SOM, and acomparison to other object-oriented languages is provided to aid inunderstanding the invention.

INTRODUCTION TO OBJECT-ORIENTED PROGRAMMING

A new development in the software community is Object-OrientedProgramming. Object-Oriented Programming Languages (OOPL) are being usedthroughout the industry, Object-Oriented Databases (OODB) are startingwidespread interest, even Object-Oriented Design and Analysis (OODA)tools are changing the way people design and model systems.

Object-Oriented Programming is best understood in contrast to its closecousin, Structured Programming. Both attempt to deal with the same basicissue, managing the complexity of ever more complex software systems.Structured Programming models a system as a layered set of functionalmodules. These modules are built up in a pyramid like fashion, eachlayer representing a higher level view of the system. StructuredProgramming models the system's behavior, but gives little guidance tomodeling the system's information.

Object-Oriented Programming models a system as a set of cooperatingobjects. Like Structured Programming, it tries to manage the behavioralcomplexity of a system. Object-Oriented Programming, however, goesbeyond Structured Programming in also trying to manage the informationalcomplexity of a system.

Because Object-Oriented Programming models both the behavioral andinformational complexity of a system, the system tends to be much betterorganized than if it was simply well "structured". BecauseObject-Oriented systems are better organized, they are easter tounderstand, debug, maintain, and evolve. Well organized systems alsolend themselves to code reuse,

Object-Oriented Programming envisions the dual issues of managinginformational and behavioral complexity as being closely related. Itsbasic unit of organization is the object. Objects have some associateddata, which are referred to as an object's state, and a set ofoperations, which are referred to as an object's methods. A method isimplemented by a subroutine. A class is a general description of anobject, which defines the data representative of an object's state, andthe methods for supporting the object.

OBJECT-ORIENTED PROGRAMMING IN C

Before examining SOM, consider Object-Oriented Programming in C; thiswill lead us naturally into the SOM philosophy. Consider a datastructure definition containing information related to a generic stack.The data structure encompasses a series of functions designed to operateon a stack structure. Given a basic stack definition, multiple instancesof this data structure may be declared within our program.

A generic stack definition, in C, appears below:

    ______________________________________                                        struct stackType {                                                             void *stackArray[STACK.sub.-- SIZE];                                          int stackTop;                                                                };                                                                            typedef struct stackType Stack;                                               ______________________________________                                    

A definition of a generic stack function appears next:

    ______________________________________                                        Stack *create( ); /* malloc and initialize a                                             new stack. */                                                      void *pop(        /* Pop element off stack.                                              */                                                                  Stack *thisStack);                                                           void push(        /* Push onto stack. */                                       Stack *thisStack,                                                             void *nextElement);                                                          ______________________________________                                    

Most C programmers can imagine how such functions would be written. The<Push()> function, for example, appears below.

    ______________________________________                                        void push(Stack *thisStack, void *nextElement)                                 thisStack->stackArray[thisStack->stackTop] =                                 nextElement;                                                                   thisStack->stackTop++;                                                       }                                                                             ______________________________________                                    

A client program might use this stack to, say, create a stack of wordsneeding interpretation:

    ______________________________________                                        main( )                                                                        Stack *wordStack;                                                             char *subject = "Emily";                                                      char *verb = "eats";                                                          char *object = "ice cream";                                                   char *nextWord;                                                               wordStack = create( );                                                        push(wordStack, object);                                                      push(wordStack, verb);                                                        push(wordStack, subject);                                                     /* . . . */                                                                   while (nextWord = pop(wordStack)) {                                            printf("%s n", nextWord);                                                     /* . . . */                                                                  }                                                                            }                                                                             ______________________________________                                    

This example can be used to review Object-Oriented Programming. A classis a definition of an object. The definition includes the data elementsof the object and the methods it supports. A <stack> is an example of aclass. A stack contains two data elements (<stackArray> and <stackTop>),and supports three methods, <create()>, <push()>, and <pop()>. A methodis like a function, but is designed to operate on an object of aparticular class. An object is a specific instance, or instantiation, ofa class. The object <wordStack> is an object of class <Stack>, or<wordStack> is an instance of a stack.

Every method requires a specific object on which it is to operate. Thisobject is called a target object, or sometimes a receiving object.Notice that each method (except <create()>) takes as its first parametera pointer to the target object. This is because a program may have manyobjects of a given class, and each are potential targets for a classmethod.

There are three important advantages of this type of organization.First, generic concepts are developed which can be reused in othersituations in which similar concepts are appropriate. Second,self-contained code is developed, which can be fully tested before it isfolded into our program. Third, encapsulated code is developed in whichthe internal details are hidden and of no interest to the client. Aclient <main()> program need know nothing about the <Stack> class otherthan its name, the methods it supports, and the interfaces to thesemethods.

COMPARISON TO C++

Another beneficial comparison is between SOM and the most widespreadObject-Oriented programming language, C++. SOM has many similarities toC++. Both support class definitions, inheritance, and overridden methods(called virtual methods in C++). Both support the notion ofencapsulation. But whereas C++ is designed to support stand-aloneprogramming efforts, SOM is focused on the support of commercial qualityclass libraries. Most of the differences between SOM and C++ hinge onthis issue. C++ class libraries are version dependent, while SOM classlibraries are version independent. When a new C++ class library isreleased, client code has to be fuzzy recompiled, even if the changesare unrelated to public interfaces.

C++ supports programming in only one language, C++. SOM is designed tosupport many languages. Rather than a language, SOM is a system fordefining, manipulating, and releasing class libraries. SOM is used todefine classes and methods, but it is left up to the implementor tochoose a language for implementing methods without having to learn a newlanguage syntax.

C++ provides minimal support for implementation hiding, orencapsulation. C++ class definitions, which must be released to clients,typically include declarations for the private data and methods. In SOM,the client never has to focus on these implementation details. Theclient need see only the <.sc> files, which contains only publicinformation. C++ also provides a limited method resolution function. SOMoffers several alternatives, such as offset method resolution, namelookup resolution, and dispatch resolution.

One other interesting difference between SOM and C++ is in its notion ofclass. In C++, the class declaration is very similar to a structuredeclaration. It is a compile-time package with no characteristics thathave significance at runtime. In SOM, the class of an object is anobject. The class object is itself an instantiation of another class,called the metaclass. The class object supports a host of useful methodswhich have no direct parallels C++, such as <somGetName()>,<somGetParent()>, and <somFindMethod()>.

INTRODUCTION TO SOM

OS/2 2.0 includes a language-neutral Object-Oriented programmingmechanism called SOM (for System Object Model). Although it possible towrite Object-Oriented programs in traditional languages, such as thestack example, SOM is specifically designed to support the new paradigmand to be used with both procedural (or non Object-Oriented) languagesand Object-Oriented languages.

An important requirement of Object-Oriented programming is codereusability. Typically, code reusability is achieved through the use ofclass libraries. Today's library technology is limited in that classlibraries are always language specific. A C++ library cannot be used bya Smalltalk programmer and a Smalltalk programmer cannot utilize a c++library. Clearly it is necessary to create a language-neutral objectmodel, which can be used to create class libraries usable from anyprogramming language, procedural or Object-Oriented.

SOM introduces three important features lacking in most procedurallanguages. These are encapsulation, inheritance, and polymorphism(referred to here as "override resolution") Inheritance refers to atechnique of specifying the shape and behavior of a class (called asubclass) as incremental differences from another class (called theparent class or superclass).

Encapsulation refers to hiding implementation details from clients. Thisprotects clients from making changes in an implementation which couldadversely affect the system. For example, in the stack example there wasno protection afforded to the C code. Although clients did not need toknow the internal data structures of the stack, there was no way toprevent clients from looking at such implementation details. We coulddiscourage, but not prevent, clients from writing code which used, andpossibly corrupted, internal stack data elements.

Inheritance, or class derivation, is a specific technique for developingnew classes from existing classes. This capability provides for thecreation of new classes which are more specialized versions of existingclasses. For example, we could create a <DebuggableStack>, which is likea <stack> class, but supports further debugging methods, such as<peek()> for looking at the top value and <dump()> for printing acomplete listing of the stack.

Inheritance also provides code consolidation. So, for example, a classdefining <GraduateStudent> and <UnderGraduateStudent>, can beconsolidated into a third class, <Student>. We then define<GraduateStudent> and <UnderGraduate> as more specialized classes, bothderived from the common parent <Student>.

Inheritance introduces some additional semantics. A specialized class issaid to be derived from a more generalized class. The general class iscalled the parent class, or sometimes, the base class. The specializedclass is called the child class, or sometimes, the derived class. Achild class is said to inherit the characteristics of its parent class,meaning that any methods defined for a parent are automatically definedfor a child. Thus, because <GraduateStudent> and <UnderGraduateStudent>are both derived from <Student>, they both automatically acquire anymethods declared in their common parent.

Override resolution refers to invoked methods being resolved based notonly on the name of the method, but also on a class place within a classhierarchy. This allows us to redefine methods as we derive classes. Wemight define a <printStudentInfo()> method for <Student> and thenoverride, or redefine, the method in both <UnderGraduateStudent>, and<GraduateStudent>. Override resolution resolves based on the type of thetarget object. If the target object type is a <Student>, the <Student>version of <printStudentInfo()> is invoked. If the target object type isa <GraduateStudent>, the <GraduateStudent> version of<printStudentInfo()> is invoked.

DEFINING CLASSES IN SOM

The process of creating class libraries in SOM is a three step process.The class designer defines the class interface, implements the classmethods, and finally loads the resulting object code into a classlibrary. Clients either use these classes directly, make modificationsto suit their specific purposes, or add entirely new classes of theirown.

In SOM, a class is defined by creating a class definition file. Theclass definition file is named with an extension of "csc". In its mostbasic form, the class definition file is divided into the followingsections:

1. Include Section

This section declares files which need to be included, much like theC<#include> directive.

2. Class Name and Options

This section defines the name of the class and declares various options.

3. Parent Information

This defines the parent, or base, class for this class. All classes musthave a parent. If a class is not derived from any existing classes, thenit's parent will be the SOM defined class <SOMObject>, the classinformation of which is in the file <somobj.sc>.

4. Data Section

This section declares any data elements contained by objects of thisclass. By default, data can be accessed only by methods of the class.

5. Methods Section

This section declares methods to which objects of this class canrespond. By default, all methods declared in this section ape availableto any class client. The class definition file, <student.csc>, describesa non-derived <Student> class, and is set forth below.

    ______________________________________                                        Class Definition File: <student.csc>                                          include <somobj.sc>                                                           class:                                                                         Student;                                                                     parent:                                                                        SOMObject;                                                                   data:                                                                          char id[16];    /* student id */                                              char name[32];  /* student name */                                           methods:                                                                       void setUpStudent(char *id, char *name);                                     - sets up a new student.                                                       void printStudentInfo( );                                                    - prints the student information.                                              char *getStudentType( );                                                     - returns the student type.                                                    char *getStudentId( );                                                       - returns the student id.                                                     ______________________________________                                    

HOW TO WRITE A METHOD

Class method are implemented in the class method implementation file.Each method defined in the method section of the class definition fileneeds to be implemented. They can be implemented in any language thatoffers SOM support. C is used for an exemplary language throughout thespecification. However, one of ordinary skill in the art will realizethat any programming language can be substituted. The student classmethod implementation file, <student.c>, is set forth below.

    ______________________________________                                        Class Method Implementation File: <student.c>                                 #define Student.sub.-- Class.sub.-- Source                                    #include "student.ih"                                                         static void setUpStudent(                                                       Student *somSelf, char *id, char *name)                                       StudentData *somThis =                                                         StudentGetData(somSelf);                                                     strcpy(.sub.-- id, id);                                                       strcpy(.sub.-- name, name);                                                 }                                                                             static void printStudentInfo(Student *somSelf)                                {                                                                               StudentData *somThis =                                                         StudentGetData(somSelf);                                                     printf("  Id    : %s  n", .sub.-- id);                                        printf("  Name  : %s  n", .sub.-- name);                                      printf("  Type  : %s  n",                                                   .sub.-- getStudentType(somSelf));                                             }                                                                             static char *getStudentType(Student *somSelf)                                 {                                                                               StudentData *somThis =                                                         StudentGetData(somSelf);                                                     static char *type = "student";                                                return (type);                                                              }                                                                             static char *getStudentId(Student *somSelf)                                   {                                                                               StudentData *somThis =                                                         StudentGetData(somSelf);                                                     return (.sub.-- id);                                                        }                                                                             ______________________________________                                    

Notice that the method code appears similar to standard C. First, eachmethod takes, as its first parameter, a pointer (<somSelf>) to thetarget object. This parameter is implicit in the class definition file,but is made explicit in the method implementation. Second, each methodstarts with a line setting an internal variable named <somThis>, whichis used by macros defined within the SOM header file. Third, names ofdata elements of the target object are preceded by an underscorecharacter "₋₋ ". The underscored name represents a C language macrodefined in the class header file. Fourth, methods are invoked by placingan underscore "₋₋ " in front of the method name. This underscored namerepresents a macro for message resolution and shields a programmer fromhaving to understand the details of this process.

The first parameter of every method is always a pointer to the targetobject. This is illustrated below in the method <printStudentInfo()>which invokes the method <getStudentType()> on its target object.

    ______________________________________                                        SOM compiler generated <student.c>                                            #define Student.sub.-- Class.sub.-- Source                                    #include "student.ih"                                                         static void setUpStudent(                                                       Student *somSelf, char *id, char *name)                                       StudentData *somThis =                                                         StudentGetData(somSelf);                                                   }                                                                             static void printStudentInfo(Student *somSelf)                                {                                                                               StudentData *somThis =                                                         StudentGetData(somSelf);                                                   }                                                                             /* . . . and so on for the other methods. */                                  ______________________________________                                    

MECHANICS OF USING SOM

There are a set of files involved with each class which are discussedbelow. The files have different extensions, but all have the samefilename as the class definition file, <Student> in our example. Thesefiles are described below.

STUDENT CLASS FILES

<student.csc>--This is the class definition file, as described earlier.

<student.sc>--This is a subset of the class definition file. It includesall information from the <.csc> file which is public, including commentson public elements. For the student example, <student.sc> would includeeverything from <student.csc> except the data section. This file iscreated by the SOM compiler.

<student.h>--This is a valid C header file which contains macrosnecessary to invoke public methods and access public data elements ofthe class. This file will be included in any client of the class, and iscreated by the SOM compiler. <student,ih>--Similar to <student.h>, butit contains additional information needed for implementing methods. Thisis the implementor's version of the <.h> file, and must be included inthe class methods implementation file. This file is created by the SOMcompiler and should not be edited.

<student.c>--Contains the method implementations. Initially created bythe SOM compiler and then updated by the class implementor.

BUILDING SOM CLASSES FROM OTHER CLASSES

There are two ways to use classes as building blocks for other classes.These are derivation (or inheritance) and construction.

DERIVATION

In this example, <GraduateStudent> is derived from <Student>, its base,or parent class. A derived class automatically picks up all of thecharacteristics of the base class. A derived class can add newfunctionality through the definition and implementation of new methods.A derived class can also redefine methods of its base class, a processcalled overriding. For example <GraduateStudent> adds<setUpGranduateStudent()> to those methods it inherits from <Student>.It overrides two other inherited methods, <printStudentInfo()> and<getStudentType()>. It inherits without change <setUpStudent()> and<getStudentID()> from the <Student> base class.

The class definition file for <GraduateStudent>, <graduate.csc>, is setforth below.

    ______________________________________                                        Class Definition File: <graduate.csc>                                         include <student.sc>                                                          class:                                                                         GraduateStudent;                                                             parent:                                                                        Student;                                                                     data:                                                                          char  thesis[128];                                                                            /* thesis title */                                            char  degree[16];                                                                             /* graduate degree type                                      */                                                                            methods:                                                                       override  printStudentInfo;                                                   override  getStudentType;                                                     void  setUpGraduateStudent(                                                      char *id, char *name, char *thesis, char                                    *degree);                                                                   ______________________________________                                    

The method implementation file, <graduate.c>, is shown below.

    ______________________________________                                        Class Method Implementation File: <graduate.c>                                #define GraduateStudent.sub.-- Class.sub.-- Source                            #include "graduate.ih"                                                        static void printStudentInfo(GraduateStudent                                  *somSelf)                                                                       GraduateStudentData *somThis =                                              GraduateStudentGetData(somSelf);                                                parent.sub.-- printStudentInfo(somSelf);                                      printf("  Thesis                                                                             : %s  n", .sub.-- thesis);                                     printf("  Degree                                                                             : %s  n", .sub.-- degree);                                   }                                                                             static char *getStudentType(GraduateStudent                                   *somSelf)                                                                     {                                                                               static char *type = "Graduate";                                               return (type);                                                              }                                                                             static void setUpGraduateStudent(                                               GraduateStudent *somSelf, char *id, char                                       *name,                                                                       char *thesis, char *degree)                                                 {                                                                               GraduateStudentData *somThis =                                              GraduateStudentGetData(somSelf);                                                .sub.-- setUpStudent(somSelf,id,name);                                        strcpy(.sub.-- thesis, thesis);                                               strcpy(.sub.-- degree, degree);                                             }                                                                             ______________________________________                                    

Often an overridden method will need to invoke the original method ofits parent. For example, the <printStudentInfo()> for <GraduateStudent>first invokes the <Student> version of <printStudentInfo()> beforeprinting out the <GraduateStudent> specific information. The syntax forthis is "<patent₋₋ MethodName>", as can be seen in the<printStudentInfo()> method.

A given base class can be used for more than one derivation. The class,<UnderGraduateStudent>, is also derived from <Student>. The classdefinition file, <undergrad.csc>, is set forth below.

    ______________________________________                                        Class Definition File: <undgrad.csc>                                          include <student.sc>                                                          class:                                                                         UnderGraduateStudent;                                                        parent:                                                                        Student;                                                                     data:                                                                          char  date[16];   /* graduation date */                                      methods:                                                                       override  printStudentInfo;                                                   override  getStudentType;                                                     void  setUpUnderGraduateStudent(                                                char *id, char *name, char *date);                                         ______________________________________                                    

The method implementation file, <undergrad.c>, is set forth below.

    ______________________________________                                        Class Method Implementation File: <undgrad.c>                                 #define UnderGraduateStudent.sub.-- Class.sub.-- Source                       #include "indgrad.ih"                                                         static void printStudentInfo(                                                   UnderGraduateStudent *somSelf)                                                UnderGraduateStudentData *somThis =                                            UnderGraduateStudentGetData(somSelf);                                        parent.sub.-- printStudentInfo(somSelf);                                      printf("  Grad Date  : %s  n", .sub.-- date);                               }                                                                             static char *getStudentType(UnderGraduateStudent                              *somSelf)                                                                     {                                                                               static char *type = "UnderGraduate";                                          return (type);                                                              }                                                                             static void setUpUnderGraduateStudent(                                          UnderGraduateStudent *somSelf,char *id, char                                *name, char *date)                                                            {                                                                               UnderGraduateStudentData *somThis =                                            UnderGraduateStudentGetData(somSelf);                                        .sub.-- setUpStudent(somSelf,id,name);                                        strcpy(.sub.-- date, date);                                                 }                                                                             ______________________________________                                    

The second technique for building classes is construction. Constructionrefers to a class using another class, but not through inheritance. Agood example of construction is the class <Course> which includes anarray of pointers to <Student>s. Each pointer contains the address of aparticular student taking the course. <Course> is constructed from<Student>. The class definition file for <Course>, <course.csc>, isshown below.

    ______________________________________                                        Class Definition File: <course.csc>                                           include <somobj.sc>                                                           class:                                                                         Course;                                                                      parent:                                                                        SOMObject;                                                                   data:                                                                          char      code[8];    /* course code                                                    number */                                                           char      title[32];  /* course title */                                      char      instructor[32];                                                                           /* instructor                                                     teaching */                                                         int       credit;      /* number of credits                                                                */                                               int       capacity;    /* maximum number of                                             seats */                                                            Student   StudentList[20];                                                                           /* enrolled student                                    int       enrollment;  /* number of                                                     enrolled students */                                               methods:                                                                       override  somInit;                                                            void  setUpCourse(char *code, char *title,                                     char *instructor, int credit, int capacity);                                - sets up a new course.                                                        int  addStudent(Student *student);                                           - enrolls a student to the course.                                             void  dropStudent(char *studentId);                                          - drops the student from the course.                                           void  printCourseInfo( );                                                    - prints course information.                                                  ______________________________________                                    

Often classes will want to take special steps to initialize theirinstance data. An instance of <Course> must at least initialize the<enrollment> data element, to ensure the array index starts in a validstate. The method <somInit()> is always called when a new object iscreated. This method is inherited from <SOMObject>, and can beoverridden when object initialization is desired.

This example brings up an interesting characteristic of inheritance, the"is-a" relationship between derived and base classes. Any derived classcan be considered as a base class. We say that a derived class "is-a"base class. In the previous example, any <GraduateStudent> "is-a"<Student>, and can be used anyplace we are expecting a <Student>. Theconverse is not true. A base class is not a derived class. A <Student>can not be treated unconditionally as a <GraduateStudent>. Thus,elements of the array <studentList> can point to either <Student>s, a<GraduateStudent>s, or a <UnderGraduateStudent>s.

The method implementation file for <Course>, <course.c>, is set forthbelow.

    ______________________________________                                        Class Method Implementation File: <course.c>                                  #define Course.sub.-- Class.sub.-- Source                                     #include <student.h>                                                          #include "course.ih"                                                          static void somInit(Course *somSelf)                                            CourseData *somThis = CourseGetData(somSelf);                                 parent.sub.-- somInit(somSelf);                                               .sub.-- code[0] = .sub.-- title[0] = .sub.-- instructor[0] = 0;               .sub.-- credit = .sub.-- capacity = .sub.-- enrollment = 0;                 }                                                                             static void setUpCourse(Course *somSelf, char                                 *code,                                                                          char *title, char *instructor, int credit,                                  int capacity)                                                                 {                                                                               CourseData *somThis = CourseGetData(somSelf);                                 strcpy(.sub.-- code, code);                                                   strcpy(.sub.-- title, title);                                                 strcpy(.sub.-- instructor, instructor);                                       .sub.-- credit = credit;                                                      .sub.-- capacity = capacity;                                                }                                                                             static int addStudent(Course *somSelf, Student                                *student)                                                                     {                                                                               CourseData *somThis = CourseGetData(somSelf);                                 if(.sub.-- enrollment >= .sub.-- capacity) return (-1);                       .sub.-- studentList[.sub.-- enrollment++] = student;                          return(0);                                                                  }                                                                             static void dropStudent(Course *somSelf, char                                 *studentId)                                                                   {                                                                               int i;                                                                        CourseData *somThis = CourseGetData(somSelf);                                 for(i=0; i<.sub.-- enrollment; i++)                                            if(!strcmp(studentId,                                                      .sub.-- getStudentId(.sub.-- studentList[i]))) {                                  .sub.-- enrollment--;                                                         for(i; i<.sub.-- enrollment; i++)                                              .sub.-- studentList[i] = .sub.-- studentList[i+1];                           return;                                                                      }                                                                          }                                                                             static void printCourseInfo(Course *somSelf)                                  {                                                                               int i;                                                                        CourseData *somThis = CourseGetData(somSelf);                                 printf(" %s %s  n", .sub.-- code, .sub.-- title);                             printf(" Instructor Name : %s  n",                                             .sub.-- instructor);                                                         printf(" Credit = %d, Capacity = %d,                                           Enrollment = %d  n n",                                                          .sub.-- credit, .sub.-- capacity, .sub.-- enrollment);                     printf(" STUDENT LIST:  n n");                                                for(i=0; i<.sub.-- enrollment; i++) {                                          .sub.-- printStudentInfo(.sub.-- studentList[i]);                             printf(" n");                                                                }                                                                           }                                                                             ______________________________________                                    

Notice in particular the method <printCourseInfo()>. This method goesthrough the array <studentList> invoking the method <printStudentInfo()>on each student. This method is defined for <Student>, and thenoverridden by both <GraduateStudent> and <UnderGraduateStudent>. Sincethe array element can point to any of these three classes, it isimpossible at compile time to determine what the actual type of thetarget object is, only that the target object is either a <Student> orsome type derived from <Student>. Since each of these classes defines adifferent <printStudentInfo()> method, it is impossible to determinewhich of these methods will be invoked with each pass of the loop. Thisis all under the control of override resolution.

THE SOM CLIENT

To understand how a client might make use of these four classes in aprogram, an example is presented below in the file <main.c>. The exampleilluminates object instantiation and creation in SOM, and how method areinvoked.

    ______________________________________                                        SOM client code:  <main.c>                                                    #include <student.h>                                                          #include <course.h>                                                           #include <graduate.h>                                                         #include <undgrad.h>                                                          main( )                                                                        Course *course = CourseNew( );                                                GraduateStudent *jane = GraduateStudentNew( );                                UnderGraduateStudent *mark =                                                    UnderGraduateStudentNew( );                                                 .sub.-- setUpCourse(course, "303", "Compilers ",                                "Dr. David Johnson", 3, 15);                                                .sub.-- setUpGraduateStudent(jane,"423538","Jane                                Brown",                                                                       "Code Optimization","Ph.D.");                                               .sub.-- setUpUnderGraduateStudent(mark,"399542",                                "Mark Smith", "12/17/92");                                                  .sub.-- addStudent(course, jane);                                             .sub.-- addStudent(course, mark);                                             .sub.-- printCourseInfro(course);                                            }                                                                             ______________________________________                                    

A class is instantiated with the method <classNameNew()>, which isautomatically defined by SOM for each recognized class. Methods areinvoked by placing an underscore "₋₋ " in front of the method name. Thefirst parameter is the target object. The remaining parametersilluminate additional information required by the method. When run, theclient program gives the output shown below.

CLIENT PROGRAM OUTPUT

303 Compilers

Instructor Name: Dr. David Johnson

Credit=3, Capacity=15, Enrollment=2

    ______________________________________                                        STUDENT LIST:                                                                         Id      : 423538                                                              Name    : Jane Brown                                                          Type    : Graduate                                                            Thesis  : Code Optimization                                                   Degree  : Ph.D.                                                               Id      : 399542                                                              Name    : Mark Smith                                                          Type    : UnderGraduate                                                       Grad Date                                                                             : 12/17/92                                                    ______________________________________                                    

The client program output illustrates the override resolution at work inthe different styles of displaying <UnderGraduate>s and<GraduateStudent>s. A <course> thinks of itself as containing an arrayof <Student>s, and knows that any <Student> responds to a<printStudentInfo()> method. But the <printStudentInfo()> method that an<UnderGraduate> responds to is different than the <printStudentInfo()>method that a <GraduateStudent> responds to, and the two methods givedifferent outputs.

SOM OBJECT MODEL

FIG. 2 is a drawing of a basic SOM data structure in accordance with thesubject invention. Label 210 is a state data structure for a particularobject. The first full word at label 220 contains the address of theobject's method procedure table label 240. The rest of the state datastructure set forth at label 230 contains additional informationpertaining to the object. The method procedure table set forth at label240 contains the address of the class object data structure 245 andaddresses of various methods for the particular object 250 and 260. Theaddress at 245 points to the class object data structure 248. Allobjects that are of the same class as this object also contain anaddress that points to this method procedure table diagrammed at label240. Any methods inherited by the objects will have their methodprocedure addresses at the same offset in memory as they appear in themethod procedure table as set forth at label 240 of the ancestor classfrom which it is inherited.

Addresses of the blocks of computer memory containing the series ofinstructions for two of the method procedures are set forth at labels250 and 260. Labels 270 and 280 represent locations in a computer memorycontaining the series of instructions of particular method procedurespointed to by the addresses represented by labels 250 and 260.

THE SOM BASE CLASSES

Much of the SOM Object Model is implemented by three classes that arepart of the basic SOM support. Briefly these classes are:

SOMObject--This class is the root class of all SOM classes. Any classmust be descended from SOMObject. Because all classes are descended fromSOMObject they all inherit and therefore support the methods defined bySOMObject. The methods of SOMObject like the methods of any SOM classcan be overridden by the classes descended from SOMObject,

SOMClass--This class is the root meta class for all SOM meta classes. Ameta class is a class whose instances are class objects. SOMClassprovides the methods that allow new class objects to be created,

SOMClassMgr--This class is used to create the single object in a SOMbased program that manages class objects.

The three SOM base classes are defined below.

SOMObject

This is the SOM root class, all SOM classes must be descended from<SOMObject>. <SOMObject> has no instance data so there is noper-instance cost to being descended from it.

SOMObject has the following methods:

Method: somInit

Parameters: somSelf

Returns: void

Description:

Initiates <self>. As instances of <SOMObject> do not have any instancedata there is nothing to initialize and you need not call this method.It is provided to induce consistency among subclasses that requireinitialization.

<somInit> is called automatically as a side effect of object creation(i.e., by <somNew>). If this effect is not desired, you can supply yourown version of <somNew> (in a user-written metaclass) which does notinvoke <somInit>.

When overriding this method you should always call the parent classversion of this method BEFORE doing your own Initialization.

Method: somUninit

Parameters: somSelf

Returns: void

Description:

(Un-initialize self) As instances of <SOMObject> do not have anyinstance data there is nothing to un-initialize and you need not callthis method. It is provided to induce consistency among subclasses thatrequire un-initialization.

Use this method to clean up anything necessary such as dynamicallyallocated storage. However this method does not release the actualstorage assigned to the object instance. This method is provided as acomplement to <somFree> which also releases the storage associated witha dynamically allocated object. Usually you would Just call <somFree>which will always call <somUninit>. However, in cases where <somRenew>(see the definition of <SOMClass>) was used to create an objectinstance, <somFree> cannot be called and you must call <somUninit>explicitly.

When overriding this method you should always call the parentclassversion of this method AFTER doing your own un-initialization.

Method: somFree

Parameters: somSelf

Returns: void

Description:

Releases the storage associated with <self>, assuming that <self> wascreated by <somNew> (or another class method that used <somNew>). Nofuture references should be made to <self>. Will call <somUninit> on<self> before releasing the storage.

This method must only be called on objects created by <somNew> (see thedefinition of <somClass>) and never on objects created by <somRenew>.

It should not be necessary to override this method. (Override<somUninit> instead.)

Method: somGetClassName

Parameters: somSelf

Returns: Zstring

Description:

Returns a pointer to this object's class's name, as a NULL terminatedstring. It should not be necessary to override this method as it justinvokes the class object's method (<somGetName>) to get the name.

Method: somGetClass

Parameters: somSelf

Returns: SOMClass *

Description:

Returns this object's class object.

Method: somGetSize

Parameters: somSelf

Returns: integer4

Description:

Returns the size of this instance in bytes.

Method: somRespondsTo

Parameters: somSelf, somId Mid

Returns: int

Description:

Returns 1 (true) if the indicated method is supported by this object'sclass and 0 (false) otherwise.

Method: somIsA

Parameters: somSelf, SOMClass *Aclassobj

Returns: int

Description:

Returns 1 (true) if <self>'s class is a descendent class of <Aclassobj>and 0 (false) otherwise. Note: a class object is considered to bedescended from itself for the purposes of this method.

Method: somIsInstanceOf

Parameters: somSelf, SOMClass *Aclassobj

Returns: int

Description:

Returns 1 (true) if <self> is an instance of the specified <Aclassobj>and 0 (false) otherwise.

SOMObject methods that support dynamic object models, These methods makeit easier for very dynamic domains to bind to the SOM object protocolboundary. These methods determine the appropriate method procedure andthen call it with the arguments specified. The default implementation ofthese methods provided in this class simply lookup the method by nameand call it. However, other classes may choose to implement any form oflookup they wish. For example, one could provide an implementation ofthese methods that used the CLOS form of method resolution. For domainsthat can do so it will generally be much faster to invoke their methodsdirectly rather than going through a dispatch method. However, allmethods are reachable through the dispatch methods. SOM provides a smallset of external procedures that wrap these method calls so that thecaller need never do method resolution.

These methods are declared to take a variable length argument list, butlike all such methods the SOM object protocol boundary requires that thevariable part of the argument list be assembled into the standard,platform-specific, data structure for variable argument lists before themethod is actually invoked. This can be very useful in domains that needto construct the argument list at runtime. As they can invoke methodswithout being able to put the constructed arguments in the normal formfor a call. This is helpful because such an operation is usuallyimpossible in most high level languages and platform-specific assemblerlanguage routines would have to be used.

Note: Different methods are defined for different return value shades.This avoids the memory management problems that would arise in somedomains if an additional parameter was required to carry the returnvalue. SOM does not support return values except for the four familiesshown below. Within a family (such as integer) SOM only supports thelargest member.

Method: somDispatchV

Parameters: somSelf, somId methodId, somId descriptor, . . .

Returns: void

Description:

Does not return a value.

Method: somDispatchL

Parameters: somSelf, somId methodId, somId descriptor

Returns: integer4

Description:

Returns a 4 byte quantity in the normal manner that integer data isreturned. This 4 byte quantity can, of course, be something other thanan integer.

Method: somDispatchA

Parameters: somSelf, somId methodId, somId descriptor

Returns: void *

Description:

Returns a data structure address in the normal manner that such data isreturned.

Method: somDispatchD

Parameters: somSelf, somId methodId, somId descriptor

Returns: float8

Description:

Returns a 8 byte quantity in the normal manner that floating point datais returned.

SOMObject Methods That Support Development

The methods in this group are provided to support program development.They have been defined in such a way that most development contexts willfind them easy to exploit. However, some contexts may need to customizetheir I/O facilities. We have attempted to allow this customization in avery portable manner, however not all contexts will be able to performthe customization operations directly because they require passingfunction parameters. We chose this approach because it allows greatplatform-neutral flexibility and we felt that any provider ofdevelopment support would find it reasonable to provide thecustomizations necessary for her/his specific development environment.

The chosen approach relies on a character output routine. An externalvariable, <SOMOutCharRoutine>, points to this routine. The SOMenvironment provides an implementation of this routine that should workin most development environments (it writes to the standard outputstream), A development context can, however, assign a new value to<SOMOutCharRoutine> and thereby redefine the output process. SOMprovides no special support for doing this assignment.

Method: somPrintSelf

parameters: somSelf

Returns: SOMAny *

Description:

Uses <SOMOutCharRoutine> to write a brief string with identifyinginformation about this object. The default implementation just gives theobject's class name and its address in memory, <self> is returned.

Method: somDumpSelf

Parameters: somSelf, int level

Returns: void

Description:

Uses <SOMOutCharRoutine> to write a detailed description of this objectand its current state. <level> indicates the nesting level fordescribing compound objects it must be greater than or equal to zero.All lines in the description will be preceded by <2*level> spaces.

This routine only actually writes the data that concerns the object as awhole, such as class, and uses <somDumpSelfInt> to describe the object'scurrent state This approach allows readable descriptions of compoundobjects to be constructed.

Generally it is not necessary to override this method, if it isoverridden it generally must be completely replaced.

Method: somDumpSelfInt

Parameters: somSelf, int level

Returns: void

Description:

Uses <SOMOutCharRoutine> to write out the current state of this object.Generally this method will need to be overridden. When overriding it,begin by calling the parent class form of this method and then write outa description of your class's instance data. This will result in adescription of all the object's instance data going from its rootancestor class to its specific class.

SOMClass

This is the SOM metaclass. That is, the instances of this class areclass objects. When the SOM environment is created one instance of thisclass with the external name <SOMClassClassData.classObject> is created.This class object is unique because it is its own class object. That is,SOMClassClassData.classObject=₋₋somGetClass(SOMClassClassData.classObject). This class introduces thesomNew and somRenew methods that are used to create new instances of SOMobjects. somNew applied to <SOMClassClassData.classObject> produces anew class object which can then be initialized to become a particularnew class. SOMClass can be subclassed just like any SOM class. Thesubclasses of SOMClass are new metaclasses and can generate classobjects with different implementations than those produced by<SOMClassClassData.classObject>. SOMClass is descended from SOMObject.SOMClass defines the following methods.

Method: somNew

Parameters: somSelf

Returns: SOMAny *

Description:

Make an instance of this class. When applied to<SOMClassClassData.classObject>, or any other metaclass object, thiswill produce a new class object; when applied to a regular class objectthis will produce an instance of that class.

Method: somRenew

Parameters: somSelf, SOMAny *obj

Returns: SOMAny *

Description:

Make an instance of this class, but use the space pointed to by <obj>rather than allocating new space for the object. Note: no test is madeto insure that <obj> points to enough space. <obj> is returned, but itis now a pointer to a valid, initialized, object.

Method: somInitClass

Parameters: somSelf, Zstring className, SOMAny *parentClass, integer4instanceSize, int maxStaticMethods, integer4 majorVersion, integer4minorVersion

Returns: void

Description:

Initialize <self>.

<parentClass> is the parent (or parent class) of this class, it may beNULL in which case it defaults to SOMObject (actuallySOMObjectClassData.classObject the class object for SOMObject). If aparent class is specifed then it must have already been created as apointer to its class object is required.

<instanceSize> should be just the space needed for this class, it is notnecessary to consider the parent class's (if any) space requirements.

<maxStaticMethods> should be just the static methods defined by thisclass, it is not necessary to consider the parent class's methods (ifany), even if they are overridden in this class.

<majorVersion> indicates the major version number for thisimplementation of the class definition, and <minorVersion> indicates theminor version number.

Method: somClassReady

Parameters: somSelf

Returns: void

Description:

This method is invoked when all of the static initialization for theclass has been finished. The default implementation simply registers thenewly constructed class with the SOMClassMgr. Metaclasses may overridethis method to augment the class construction sequence in any way thatthey wish.

Method: somGetName

Parameters: somSelf

Returns: Zstring

Description:

Returns this object's class name as a NULL terminated string.

Method: somGetParent

Parameters: somSelf

Returns: SOMClass w

Description:

Returns the parent class of self if one exists and NULL otherwise.

Method: somGetClassData

Parameters: somSelf

Returns: somClassDataStructure *

Description:

Returns a pointer to the static <className>ClassData structure.

Method: somSetClassData

Parameters: somSelf, somClassDataStructure *cds

Returns: void

Description:

Sets the class' pointer to the static <className>ClassData structure.

Method: somDescendedFrom

Parameters: somSelf, SOMClass *Aclassobj

Returns: int

Description:

Returns 1 (true) if <self> is a descendent class of <Aclassobj> and 0(false) otherwise. Note: a class object is considered to be descendeditself for the purposes of this method.

Method: somCheckVersion

Parameters: somSelf, integer4 majorVersion, integer4 minorVersion

Returns: int

Description:

Returns 1 (true) if the implementation of this class is compatible withthe specified major and minor version number and false (0) otherwise. Animplementation is compatible with the specified version numbers if ithas the same major version number and a minor version number that isequal to or greater than <minorVersion>. The major, minor version numberpair (0,0) is considered to match any version. This method is usuallycalled immediately after creating the class object to verify that adynamically loaded class definition is compatible with a usingapplication.

Method: somFindMethod

Parameters: somSelf, somId methodId, somMethodProc **m

Returns: int

Description:

Finds the method procedure associated with <methodId> for this class andsets <m> to it. 1 (true) is returned when the method procedure directlycallable and 0 (false) is returned when the method procedure is adispatch function.

If the class does not support the specified method then <m> is set toNULL and the return value is meaningless.

Returning a dispatch function does not guarantee that a class supportsthe specified method; the dispatch may fail.

Method: somFindMethodOk

Parameters: somSelf, somId methodId, somMethodProc **m

Returns: int

Description:

Just like <somFindMethod> except that if the method is not supportedthen an error is raised and execution is halted.

Method: somFindSMethod

Parameters: somSelf, somId methodId

Returns: somMethodProc *

Description:

Finds the indicated method, which must be a static method defined forthis class, and returns a pointer to its method procedure. If the methodis not defined (as a static method or at all) for this class then a NULLpointer is returned.

Method: somFindSMethodOk

Parameters: somSelf, somId methodId

Returns: somMethodProc *

Description:

Just like <somFindSMethod> except that an error is raised if the methodis not defined for this class.

Method: somSupportsMethod

Parameters: somSelf, somId Mid

Returns: int

Description:

Returns 1 (true) if the indicated method is supported by this class and0 (false) otherwise.

Method: somGetNumMethods

Parameters: somSelf

Returns: int

Description:

Returns the number of methods currently supported by this class,including inherited methods (both static and dynamic).

Method: somGetInstanceSize

Parameters: somSelf

Returns: integer4

Description:

Returns the total size of an instance of <self>. All instances of <self>have the same size.

Method: somGetInstanceOffset

Parameters: somSelf

Returns: integer4

Description:

Return the offset in the body part of this object for the instance databelonging to this class,

Method: somGetInstancePartSize

Parameters: somSelf

Returns: integer4

Description:

Returns the size in bytes of the instance data required for this class.This does not include the instance data space required for this class'ancestor or descendent classes.

Method: somGetNumStaticMethods

Parameters: somSelf

Returns: int

Description:

Returns the number of static methods that this class has. This is usedby a child class in initializing its method table.

Method: somGetPClsMtab

Parameters: somSelf

Returns: somMethodTab *

Description:

Returns a pointer to the method table of this class's parent class. Ifthis class is a root class (SOMObject) then NULL is returned.

Method: somGetClassMtab

Parameters: somSelf

Returns: somMethodTab *

Description:

Returns a pointer to the method table of this class.

Method: somAddStaticMethod

Parameters: somSelf, somId methodId, somId methodDescriptor,somMethodProc *method, somMethodProc *redispatchStub, somMethodProc*applyStub

Returns: somMOffset

Description;

Adds/overrides the indicated method, returns the value that should beused to set the offset value in the class data structure for this methodname.

<methodDescriptor> is a somId for a string describing the callingsequence to this method as described in <somcGetNthMethodInfo> definedin the SOMObject class definition.

<method> is the actual method procedure for this method.

<redispatchStub> is a procedure with the same calling sequence as<method> that re-dispatches the method to one of this class's dispatchfunctions.

<applyStub> is a procedure that takes a standard variable argument listdata structure applies it to its target object by calling <method> witharguments derived from the data structure. Its calling sequence is thesame as the calling sequence of the dispatch methods defined inSOMObject. This stub is used in the support of the dispatch methods usedin some classes. In classes where the dispatch functions do not needsuch a function this parameter may be null.

Method: somOverrideSMethod

Parameters: somSelf, somId methodId, somMethodProc *method

Returns: void

Description:

This method can be used instead of <somAddStaticMethod> or<somAddDynamicMethod> when it is known that the class' parent classalready supports this method. This call does not require the methoddescriptor and stub methods that the others do.

Method: somGetMethodOffset

Parameters: somSelf, somId methodId

Returns: integer4

Description:

Returns the specified method's offset in the method procedure tableassuming this is a static method, returns 0 if it was not. This value isused to set the offset value in this class data structure. It shouldonly be necessary to use this method when a class used to define amethod that it now inherits.

Method: somGetApplyStub

Parameters: somSelf, somId methodId

Returns: somMethodProc *

Description:

Returns the apply stub associated with the specified method. NULL isreturned if the method is not supported by this class. An apply stub aprocedure that is called wlth a fixed calling sequence, namely (SOMAny*self, somId methodId, somId descriptor, ap₋₋ list ap) where <ap> is avarargs data structure that contains the actual argument list to bepassed to the method. The apply stub forwards the call to its associatedmethod and then returns any result produced by the method.

SOMClassMgr

SOMClassMgr is descended from SOMObject. SOMObject defines the followingmethods:

Method: somFindClsInFile

Parameters: somSelf, somId classId, int majorVersion, tnt minorVersion,Zstring file

Returns: SOMClass *

Description:

Returns the class object for the specified class. This may result indynamic loading. If the class already exists <file> is ignored,otherwise it is used to locate and dynamically load the class. Values of0 for major and minor version numbers bypass version checking.

Method: somFindClass

Parameters: somSelf, somId classId, int majorVersion, int minorVersion

Returns: SOMClass *

Description:

Returns the class object for the specified class. This may result indynamic loading. Uses somLocateClassFile to obtain the name of the filewhere the class' code resides, then uses somFindClsInFile.

Method: somClassFromId

Parameters: somSelf, somId classId

Returns: SOMClass *

Description:

Finds the class object, given its Id, if it already exists. Does notload the class. Returns NULL If the class object does not yet exist.

Method: somRegisterClass

Parameters: somSelf, SOMClass *classObj

Returns: void

Description:

Lets the class manager know that the specified class is installed andtells it where the class object is.

Method: somUnregisterClass

Parameters: somSelf, SOMClass *classObj

Returns: int

Description:

Unloads the class file and removes the class from the SOM registry.

Method: somLocateClassFile

Parameters: somSelf, somId classId, int

majorVersion, int minorVersion

Returns: Zstring

Description:

Real implementation supplied by subclasses. Default implementationreturns the class name as the file name. Subclasses may use versionnumber info to assist in deriving the file name.

Method: somLoadClassFile

Parameters: somSelf, somId classId, int

majorVersion, int minorVersion, Zstring file

Returns: SOMClass *

Description:

Loads the class' code and initialize the class object.

Method: somLoadClassFile

Parameters: somSelf, SOMClass *classObj

Returns: int

Description:

Releases the class' code and destroys the class object

Method: somGetInitFunction

Parameters: somSelf

Returns: Zstring

Description:

Supplies the name of the initialization function in the class' codefile. Default implementation returns (*SOMClassInitFuncName)().

Method: somMergeInto

Parameters: somSelf, SOMObject *targetObj

Returns: void

Description:

Merges the SOMClassMgr registry information from the receiver to<targetObj>. <targetObj> is required to be an instance of SOMClassMgr orone of its subclasses. At the completion of this operation, the<targetObj> should be able to function as a replacement for thereceiver. At the end of the operation the receiver object (which is thenin a newly uninitialized state) is freed, Subclasses that override thismethod should similarly transfer their sections of the object and passthis method to their parent as the final step. If the receiving objectis the distinguished instance pointed to from the global variableSOMClassMgrObject, SOMCLassMgrObject is then reassigned to point to<targetObj>.

MANAGING OBJECT NAMES

The subject invention improves upon past object oriented techniques ofrequiring unique external names for every method for a class byinitializing the class method table at runtime via a special procedureassociated with each class implementation and by collecting the set ofmethod offsets into a single externally named class data structure. Thisimprovement reduces the complexities of managing a large list ofexternal variables, reduces the problem of creating unique names(referred to as name mangling), reduces the memory requirements andreduces the load time of the generated execution module.

FIG. 3 is a SOM class data structure in accordance with the subjectinvention. Label 310 represents a pointer to the class object datastructure set forth in FIG. 2 at 248. Label 320 represents an offsetinto the method procedure table set forth in FIG. 2 at label 240 or intothe object's state data structure set forth in FIG. 2 at label 230.Similarly, labels 330 and 340 represent additional offsets into themethod procedure table on into its state data structure. For additionalmethods that are first defined in this class or methods that arementioned in the class release order section but defined by one of theclass' ancestor classes, or public instance variables defined by thisclass, there are similar entries in the class data structurerepresenting offsets associated with this class as signified by theelipses and "N + 1" at label 350. The additional entry is necessarybecause of the first entry represents a pointer to the class object datastructure 248 in FIG. 2.

The order of the values in the class data structure is determined by theorder of the corresponding method or public instance variable name inthe release order section of the class OIDL file. Methods or public datamembers defined in the class but not mentioned in the release ordersection are ordered after those mentioned in the release order sectionand in the order in which they appear in the class OIDL file.

OBJECT INTERFACE DEFINITION LANGUAGE (OIDL)

The invention redefines language dependent object definitions as aneutral set of information from which object support for any language isprovided. The neutral set of information is referred to as an ObjectInterface Definition Language (OIDL) definition in SOM. SOM OIDLprovides the basis for generating binding files that enable programminglanguages to use and provide SOM objects and their definitions (referredto as classes). Each OIDL file defines the complete interface to a classof SOM objects.

OIDL files come in different forms for different languages. Thedifferent forms enable a class implementer to specify additionallanguage-specific information that allows the SOM Compiler to providesupport for constructing the class. Each of these different forms sharea common core language that specifies the exact information that a usermust know to use a class. One of the facilities of the SOM Compiler isthe extraction of the common core part of a class definition. Thus, theclass implementer can maintain a language-specific OIDL file for aclass, and use the SOM Compiler to produce a language-neutral coredefinition as needed.

This section describes OIDL with the extensions to support C-languageprogramming. As indicated above, OIDL files are compiled by the SOMCompiler to produce a set of language-specific or use-specific bindingfiles.

The SOM Compiler produces seven different files for the C language.

A public header file for programs that use a class. Use of a classincludes creating instance objects of the class, calling methods oninstance objects, and subclassing the class to produce new classes.

A private header file, which provides usage bindings to any privatemethods the class might have.

An implementation header file, which provides macros and other materialto support the implementation of the class.

An implementation template, which provides an outline of the class'implementation that the class provider can then edit.

A language-neutral core definition

A private language-neutral core file, which contains private parts ofthe class interface.

An OS/2 .DEF file that can be used to package the class in the form ofan OS/2 DLL.

OIDL files can contain the following sections:

Include section;

Class section;

Release Order section;

Metaclass section;

Parent Class section;

Passthru section;

Data section; and

Methods section.

INCLUDE SECTION

This required section contains an include statement that is a directiveto the OIDL preprocessor telling the compiler where to find the classinterface definition for this class' parent class, the class' metaclassif the class specifies one, and the private interface files for anyancestor class for which this class overrides one or more of its privatemethods,

CLASS SECTION

This required section introduces the class, giving its name, attributesand optionally a description of the class as a whole.

RELEASE ORDER SECTION

This optional section contains a release order statement that forces thecompiler to build certain critical data structures with their itemsarranged in the order specified. This allows the class interface andimplementation to be evolved without requiring programs that use thisclass be recompiled.

Release order applies to all method names and public data items. If therelease order of some method or public data item is not specified, itwill default to an implementation-specific order based on its occurrencein the OIDL file. The introduction of new public data items or methodsmight cause the default ordering of other public data items or methodsto change; programs using the class would then need to be recompiled.

METACLASS SECTION

This optional section specifies the class' metaclass, giving its nameand, optionally, a description of the reason for the metaclass, or othercomments about its role in this class' interface. If a metaclass isspecified, its definition must be included in the include section. If nometaclass is specified, the metaclass of this class' parent class willbe used.

A class' metaclass can also be implicitly defined through the combineduse of the class attribute in the data section and the class attributein the method section. If either of these attributes are used, then themetaclass section must be bypassed. In this case, the implied metaclasswill be a subclass of the metaclass of the parent class.

PARENT CLASS SECTION

This required section specifies the class' parent class by indicatingthe name and optionally a description of the role of the parent class inthis class' interface.

PASSTHRU SECTION

This optional section provides blocks of code to be passed by thecompiler into various binding files. The contents of the passedinformation are ignored by the compiler. Even comments contained inpassthru lines are processed without modification.

DATA SECTION

This optional section lists the instance variables for this class. Thissection is generally present only in the language specific version ofthe class interface definition (a .CSC file). However, it must bepresent in the public form of the class interface definition if theclass contains public instance variables. ANSI C syntax is used todescribe these variables.

METHOD SECTION

This optional section lists the methods supported by this class. ANSI Cfunction-prototype syntax is used to define the calling sequence to eachmethod.

SOM COMPILER

The SOM Compiler translates the OIDL source definition of a SOM classinto a set of bindings appropriate for a particular programminglanguage. The SOM Compiler supplied with the OS/2.0 toolkit produces acomplete set of bindings for the C programming language.

The compiler operates in two phases--a precompile phase and an emissionphase. In the first phase a precompiler reads and analyzes auser-supplied class definition and produces intermediate output filescontaining binary class information, comments and passthru lines. In thesecond phase, one or more emitter programs run to produce theappropriate language binding files. Two additional programs serve aspreprocessors for the SOM precompiler phase. The sequencing andexecution of all of these programs is directed by the SOM Compiler.

The output from the emitters, plus user-supplied logic for the class'methods, are subsequently compiled by the C compiler and linked by theOS/2 linker to create a loadable module. Loadable modules can bepackaged in self-contained files or placed in a DLL so the class can beused from many programs.

Referring to FIG. 4, control commences at terminal 400 and flowsdirectly into function block 404 where a SOM language neutral objectinterface definition (OIDL) 402 is input to the SOM OIDL compiler 406.The SOM OIDL compiler parses the object definitions in OIDL into acanonical form 406 to simplify the code generation process as input tothe target language emitter 410. The language emitter 410 generateslanguage bindings 414 which include the class data structure depicted inFIG. 3. Control flows to the language compiler shown in function block420 which receives additional inputs from the language applications 416and the SOM bindings 412. The language compiler could be a c, Fortran,Cobol or other compiler depending on user preference. Output from thelanguage compiler is an object file 422 which can be link edited withthe SOM runtime library for subsequent execution.

FIG. 5 is a flowchart depicting a link, load and execution of anapplication using SOM objects in accordance with the subject invention.Processing commences at terminal 500 and immediately flows into functionblock 530 for a dynamic link and load of the SOM objects 510 created inFIG. 4 at label 422 and the SOM run time library 520. Then, at functionblock 540, the application is started, which invokes the creation ofnecessary classes and objects as set forth in function block 550 anddetailed in FIGS. 6, 7, 8, 9 and 10. Finally, the application isexecuted as shown in function block 560 and control is terminated atterminal block 570.

VERSION INDEPENDENCE FOR OBJECT ORIENTED PROGRAMS

This aspect of the invention generally relates to improvements in objectoriented applications and more particularly solving problems arisingfrom the independent evolution of object definition libraries and thecomputer applications that use them.

The version independence processing isolates the executable binary formof computer applications that use object definition libraries (alsocalled object class libraries) from certain changes in theimplementations or specification of the object definitions thatnaturally arise during the lifecycle of the libraries. Specifically, thefollowing changes can be made to an object definition withoutcompromising its use by the unmodified executable binary form of acomputer application which dynamically loads the object definition eachtime the application is executed:

1) add new methods to an object definition;

2) move the point of definition for a method from a child class to itsparent class;

3) add to, delete from, or otherwise change the public instance dataassociated with an object definition; and

4) insert a new class definition into a class hierarchy.

This processing is accomplished by the operation of an algorithm in thememory of a processor employing several techniques as follows. Methodand instance offset are removed from application binary images. Instatic object models, such as the one defined in C++, an offset (aninteger number) into a method procedure table is used to select a methodprocedure for each particular method name. The offset depends on thenumber and order of the methods of the class the method is defined inand the number of methods defined by its ancestors.

This approach has the benefit of being a very fast form of methodresolution. However, in the prior apt object models have placed theseoffsets in the binary images of the applications that used a particularobject class, resulting in the requirement to recompile the applicationwhenever the offsets required a change.

In SOM, the offsets associated with methods are collected into a singlememory data structure for each class, called the class data structure,detailed in the discussion of FIG. 3. This data structure is given anexternal name and its contents are referred to in applications. Eachclass data structure is initialized to contain the appropriate offsetvalues when a class object is initialized as detailed in FIG. 10. Thuseach time an application is executed all the offset values arerecalculated based on the current definitions of the classes used by theapplication.

Note that any references in an application's binary images to the valuesstored in the class data structure contain offsets. However, theseoffsets can remain constant across the four kinds of changes enumeratedabove. This is because the class data structure only contains offsetsfor the methods defined in a particular class, not for offsets ofmethods inherited by the class. Thus, new methods added to a class canhave their offsets added at the end of the class data structure withoutdisturbing the positions of the offset values for methods that werealready defined in the class.

The SOM Object Interface Definition Language (OIDL) contains a ReleaseOrder Section, discussed in the section titled "SOM Object Model" above.The release order section of OIDL allows the class implementor to insurethat new method offset values are added after the method offset valuesfor methods already defined in a class. The release order section in anOIDL file also causes an entry to be retained in a class data structureif one of the methods defined in the class is moved to a parent class ashighlighted in FIG. 3. This entry is then initialized from the parentoffset value by a simple assignment statement that the OIDL compileradds to the logic initializing the class data structure as described inFIG. 10.

A similar problem arises with public instance data. An application thataccesses a public instance variable contained in one of theapplication's object's state data structure must do so via a offset intothe object's state data structure. In the prior art, this offset wascontained in application's binary image. If this technique is employed,then the application's binary image must be regenerated (viarecompilation) any time the offset changes due to a change in the sizeof one or more of the object's ancestor classes' instance datarequirements or due to changes in the object's own instance data layout.

In SOM this problem is solved by putting the offset for each public datavariable in the class data structure detailed in FIG. 3 and the ensuingdiscussion. Each class data structure is initialized to contain theappropriate offset values when the class object is initialized asdetailed in FIGS. 7 and 13. Thus, each time an application is executedall the offset values are recalculated based on the current definitionsof the classes used by the application.

REMOVE OBJECT STATE DATA STRUCTURE SIZES FROM APPLICATIONS' BINARYIMAGES

When new instances of objects are created, a correct amount of computermemory must be allocated to hold the object's state data structure. Inthe prior art, the size of this block of memory was contained in anapplication's binary image. If this technique is employed, then theapplication's binary image must be regenerated (via recompilation) anytime the size of the object's state data structure changes. In SOM, thisvalue is available via a call to the object's class object and thereforeneed not be contained in an application's binary image.

The techniques described above allow each of the four changes previouslyhighlighted to occur with respect to class definitions used by anapplication without requiring that the application's binary image to beregenerated.

FIG. 6 is a flowchart depicting the creation of a new SOM class inaccordance with the subject invention. Control commences at terminal 600which flows immediately into a test for a correct version number atdecision block 610 where a check is performed to verify the correctnessof the version number. If an incorrect version number is detected, thena message is displayed in output block 612 and control is terminated atterminal block 614. If a correct version number is detected, thenanother test is performed at decision block 620 to determine if the SOMclass exists. If the SOM class exists, then processing is returned atterminal block 622.

If the SOM class does not exist at decision block 620, then a test isperformed at decision block 630 to determine if the SOM runtimeenvironment is active. If it is not active, then the SOM runtimeenvironment is invoked at function block 632. Whether the SOMenvironment was initially present or not, control then flows to decisionblock 660 to check for an error in the SOM environment at decision block660. If an error is detected, then an appropriate message is presentedat output block 662 and processing is terminated at terminal block 644.If an error is not detected, then control passes to function block 650where a default metaclass is prepared. Next, a class is constructed infunction block 652 as detailed in FIG. 7. Finally, processing isreturned at terminal block 660.

FIG. 7 is a flowchart depicting the detailed construction of a new SOMclass in accordance with the subject invention. Control commences atterminal 700 and flows immediately into function block 710 where ageneric class object is created as detailed in FIG. 8. Next, the newgeneric class is initialized to default values at function block 720 anddetailed in FIG. 9. Then, at function block 730, the instance dataoffset is initialized for the particular new class. Control flows tofunction block 760 where the class data structure (FIG. 3) for the newclass is initialized by assigning values representing each static methodfor the new class as detailed in FIG. 10.

At function block 750, 760 and 770 the parent class is set, the classdata is initialized and the class is registered. These steps involveupdating the new class data structure as detailed in the discussion ofFIGS. 2, 10 and 13. Finally, control is returned at terminal 780.

FIG. 8 is a flowchart depicting the detailed construction of a new SOMgeneric class object in accordance with the subject invention. Controlcommences at terminal 800 and immediately flows into function block 810where memory is allocated for the object. Then, a test is performed atdecision block 820 to determine whether the memory was allocated. If anerror is detected, then an appropriate error message is displayed atoutput block 830 and processing terminated at terminal block 840. If noerror is detected, then the default values of the object are set atfunction block 850 and control is returned at terminal block 860.

FIG. 9 is a flowchart depicting the detailed initialization of a new SOMclass object in accordance with the subject invention. Control commencesat terminal 900 and immediately enters a decision block 910 and a testis performed to detect if the parent class of the new SOM class objectexists. If a parent class exists, then the parent class is initializedin function block 912. Once the parent class is initialized, then memoryfor the class name is allocated at function block 920. Next, a test isperformed again to detect if the parent class of the new SOM classobject exists at decision block 930.

If a parent class does not exist, then initial variables are set to zeroas shown in function block 932 and control passes to function block 970.If a parent class exists, then the initial variables are updated basedupon the values from the parent class in function blocks 940, 950, and960. Then, in function block 970, the version number for the class isset and error processing is performed in decision block 980. If an erroris detected, then an appropriate message is displayed at output block982 and processing terminates at terminal block 984. If no error isdetected, then control is returned at terminal block 990.

FIG. 10 is a flowchart depicting the detailed initialization of a SOMclass data structure with offset values in accordance with the subjectinvention. Control commences at terminal block 1000 and immediatelyflows into function block 1010 where a loop commences with theacquisition of the next static method. In function block 1020, the newmethod id is registered with the SOM runtime environment. Then, a testis performed to determine if the method has already been registered in aparent class in decision block 1030. If the method has been registered,then the method offset is overridden at function block 1032 and controlpasses to decision block 1070.

If the method has not been registered with any parent class, then a testis performed to determine if the method has been defined in the currentclass at decision block 1040. If the method has been defined, then theexisting offsets are employed at function block 1042 and control ispassed to decision block 1070. If the method has not been defined, thenmemory is allocated and values are initialized in function blocks 1050and 1060. In function block 1060 the offset is calculated by adding thenumber of inherited static methods to the number of inherited staticmethods processed to date by the class. Error processing is performed indecision block 1070, and if an error is detected, then an appropriatemessage is displayed at output block 1072 and processing terminates atterminal block 1074. After error processing is completed, another testis performed at decision block 1080 to determine if any additionalmethods require processing. If there are additional methods, thencontrol passes to function block 1010 for the next iteration of theloop. Otherwise, control flows to terminal 1090 where control returns.

PARENT CLASS SHADOWING

Logic for providing a dynamic insertion of a replacement parent class,referred to in object programming as a parent class shadow, is detailedin this section of the invention. This processing allows the staticallycompiled definition of what parent class is linked to a particular classat runtime to be dynamically altered during execution. The ability toinsert a new parent class into a statically compiled class hierarchyoffers more flexibility to maintain and enhance existing code after ithas appeared in binary form. It also offers a new degree of freedom forcustomizing code without access to source materials since this resultcan be achieved without recompilation.

Prior art systems have inherent limitations associated with staticallylinking derived classes and their parent classes. These limitationsinclude, computation of the size of the derived object state datastructure, initialization of the derived method procedure table, and theinability to provide access to a parent class' methods from within thederived class' methods (called parent class resolution).

The SOM object model removes these static references by having all theparent class information available at runtime through the parent classobject. Thus, when the derived class implementation needs informationabout the size of the parent class' state data structure, the addressesof the parent class' method procedures, or access to the parent class'method procedure table (to support parent class resolution) anappropriate call is placed to acquire the information from the parentclass object. The detailed processing to obtain this information aregiven in FIGS. 7, 8, 9, and 10.

SOM introduces a class manager for every SOM process. The class manageris responsible for keeping a registry of classes. The class constructioncode generated by the SOM compiler works with the class manager toestablish the relationship between a class and its parent class whenevera child class object is created. The SOM class manager is an instance ofa class which can be subclassed like any other SOM class.

Derived classes establish a connection to their parent class object bymaking calls on the SOM Class Manager object. An application designerwanting to substitute an alternate class implementation for the originalclass implementation follows the following steps:

1) Subclass SOMClassMgr providing a new set of application specificrules for determining a class object from a class name (i.e., changingthe implementations of somClassFromId, somFindClass, andsomFindClsInFile)

A simple and useful way to do this is to add a method to register ashadow class object under an existing class name and then return theshadow class object to the calling application in any subsequent callsto somClassFromId, somFindClass, or somFindClsInFile where the shadowedname is specified.

2) Before creating any derived class objects that are to have a shadowedparent class object, create an instance of the new class manager class(as described in step 1 above), initialize it from the existingSOMClassMgr instance (via the somMergeInto method), and then replace theexisting SOMClassMgr instance with the new class manager instance byoverriding the address of the existing SOMClassMgr instance in the SOMruntime.

3) Still before creating any derived class objects that are to have ashadowed parent class object, use the facilities of the applicationspecified class manager object to register the shadow class objects.

After the above three steps have been completed, derived class objectscan be created. They will be linked to the appropriate parent shadowclass objects. This will work because of the specific logic used toinitialize a class object and link to its parent class object asdepicted in FIG. 11. This logic consists of two basic steps:

1) First, a call is made to insure that the statically known parentclass object has been created. This serves two important purposes:

(a) It creates a static reference to the binary image of the staticallyknown parent class definition, thus insuring that the parent classimplementation will be linked into the binary image of the application.

(b) It insures that the at least the statically known parent classobject has been registered with the SOM class manager object before thenext step occurs.

If the statically known parent class object has already been created(say by an application following the shadowing steps discussed above)then a second attempt at this time is ignored.

2) Second, a call is made to the SOM class manager object to retrievethe address of the appropriate class object based on the name of thederived class' parent class. If the parent class has been shadowed thenthis call will return the shadow class object.

The combination of the techniques and mechanisms described aboveeffectively isolate a derived class' binary image from any dependency onthe exact class of the class object that the derived class uses toextract parent class data from.

Two restrictions must be observed when inserting a new class between achild class and its parent class. First, the insertion must beaccomplished before any instances of the child class have been created.Second, the inserted class must also be an immediate child of theoriginal parent class. Because the SOM class manager is used as anintermediary when establishing the relationships between classes at runtime, even a statically linked class can be shadowed in this manner.

FIG. 11 is a flowchart depicting the detailed parent class shadowing ofa statically defined class hierarchies in accordance with the subjectinvention. Control commences at terminal block 1100 and immediatelyflows into function block 1110 where the statically defined parent classobject is created, Next, the shadow parent class is created and used tooverride the statically defined parent class at function block 1120.Then, the child class is created as shown in function block 1130 and thechild class interrogates the SOM class manager to ascertain its current,rather than statically defined, parent class. Control returns atterminal block 1140.

REDISPATCH METHOD STUBS

A central aspect of object oriented programming is referred to as methodresolution. This processing selects a particular method given an object,the method's id and the arguments passed to the method invocation. Inmany object models, such as the one used in C++, method resolutionconsists of determining an offset into an object specific table ofprocedure entry points based on an analysis of the program's sourcecode. This type of resolution is preferred to in object models asstatic. In other object models such as the one used in Smalltalk, a moredynamic model is used that consists of using the name of the object todetermine a specific method at runtime. In object models this isreferred to as dynamic.

The invention consists of a programming mechanism called redispatchstubs to ameliorate the difference between static and dynamic models. Aredispatch stub is a small procedure with an entry point that can beplaced into a table of procedure entry points. The table of procedureentry points are used in a static object model as a substitute for theactual method entry point that is expected. The redispatch stub isgenerated automatically based on the requirements of the dynamic objectmodel. The redispatch stub converts the call generated in the staticobject model into the form necessary in the dynamic object model andsupplies any missing information in the process. Thus, if an object isaccessed from a static object model that is provided by a dynamic objectmodel, it can be represented to the static object model via a table ofentry points which each indicate a particular redispatch stub.

FIG. 12 is a flow diagram depicting the redispatch method in accordancewith the subject invention. Label 1200 is a state data structure for aparticular object. The first full word at label 1210 contains theaddress of the object's method procedure table label 1240. The rest ofthe state data structure is set forth at label 1230 contains additionalinformation pertaining to the object. The method procedure table setforth at label 1240 containing the addresses of various methods for theparticular object. All objects that are of the same class as this objectalso contain an address that points to this method procedure tablediagrammed at label 1240. Any methods inherited by the objects will havetheir method procedure addresses at the same offset in memory as theyappear in the method procedure table as set forth at label 1240 of theancestor class from which it is inherited.

In the figure, label 1250 contains a pointer to a redispatch stub 1270.A redispatch stub is a sequence of instructions that appear as a methodto a client program. However, the instructions merely convert the methodcall into a call to an object's appropriate dispatch function asillustrated at label 1260. The address at label 1260 is a pointer to theobject's dispatch function 1280. All SOM objects have a dispatchfunction. The dispatch function 1280 implements an algorithm to select aparticular method based on the parameters passed by the redispatch stub.These parameters include the method's identifier, a string describing aset of arguments passed to the identified method, and a data structurecontaining the set of arguments.

OFFSET VALUES

FIG. 13 is a flowchart depicting the detailed initialization of theoffset value in a SOM class data structure for a single public instancevariable. This logic sequence is repeated for each public instancevariable defined in a particular class (see the discussion of the OIDLData Section above). Control commences at the terminal block 1300 andimmediately flows into the function block 1310 where the offset of theinstance variable is calculated by adding the instance variable's offsetwithin this class' object state data to the offset of the beginning ofthis class' object state data within the object state data structure setforth in FIG. 2 at label 230.

The beginning of the class' object state data is determined by adding upthe sizes of each of this class' ancestor classes' object state data.Control then passes to function block 1320 when the calculated offset isstored into the position in the class data structure as determined bythe position of the public instance variable's name in the OIDL filesRelease Order Section (see the OIDL Release Order section above and FIG.3 above). Control then flows to the terminal block 1330 and the processis complete.

REDISPATCH STUBS

FIG. 14 is a flowchart depicting the detailed control flow that occurswhen a redispatch stub is employed to convert a static method call intoa dynamic method call. Control commences at the terminal block 1400 andimmediately flows into the function block 1410 where the address of theredispatch stub is determined in the normal static method resolutionmanner by getting the address stored in the object's method proceduretable at an offset contained in the appropriate class data structure atposition determined when the class was defined.

Control then passes to function block 1420 where the redispatch stub iscalled exactly like it was the real static method procedure. Functionblock 1430 depicts how the redispatch stub calls the object's dispatchmethod (using normal method resolution as described above). Theredispatch stub adds the method's identifier and descriptor to the callas required by the object's dispatch method. These values areincorporated into the redispatch function definition when it isgenerated by the SOM OIDL compiler. (Note: as detailed in the definitionof the SOMObject class above, all classes must support dispatchmethods). The object's dispatch method procedure determines which actualmethod procedure should be called using an algorithm specific to theobject's class as shown in function block 1440.

SOM provides a default implementation of such an algorithm that looksthe method's identifier up in a table contained in the object's classobject to determine the address of a method procedure. Other objectmodels might use other algorithms. Control then passes to function block1450 where the method procedure determined in block 1440 is called. Whenthe method procedure returns its return value if any is returned to theoriginal caller of the redispatch stub at terminal block 1460. Theredispatch stub allows the original static method call to be convertedto one of arbitrary dynamics without requiring any changes to theapplication program that is manipulating the object.

METHOD PROCEDURE TABLE INITIALIZATION

FIG. 15 is a flowchart depicting the detailed control flow that willproperly initialize a method procedure table for a class that may changethe association of method procedures to method during the execution ofan application using the class. Control commences at terminal block 1500and immediately flows into function block 1510 where space is allocatedfor the method procedure table. Enough space is allocated to contain anentry for the address of the class' object and each of the methodinherited or defined by the class in accordance with FIG. 7. Controlthen passes to function block 1520 where each method entry in the methodprocedure table is replaced by its redispatch stub. Redispatch stubs forinherited are determined by requesting them from the class' parentclass. Redispatch stubs for the class are generated by the SOM compilerand supplied to the class initialization procedure in the calls toregister each of the class' static method. Control then passes tofunction block 1530 where the method procedure table entries for theclass' dispatch function are replaced by the actual address of theclass' dispatch function (it is never correct to have a redispatch stubaddress in a dispatch function slot as this would result in a infiniteloop). Finally control passes to the terminal block 1540 and processingis complete.

While the invention has been described in terms of a preferredembodiment in a specific system environment, those skilled in the artrecognize that the invention can be practiced, with modification, inother and different hardware and software environments within the spiritand scope of the appended claims.

What is claimed is:
 1. A system for externally accessing computerimplemented reusable object libraries having a consistent binaryinterface, comprising:means for specifying object data and objectmethods to create an object specification; means for compiling saidobject specification to create a computer executable objectspecification in said reusable object library; and means for collectingobject data and object method addresses in said reusable object libraryinto an externally accessible data structure having a defined binaryinterface when said specified object library is loaded for execution ina computer system.
 2. The system of claim 1 wherein said means forspecifying includes means for specifying a subset of said object dataand said object methods to be externally accessible by other computerimplemented processes and wherein said means for collecting collectsonly the externally accessible object and methods.
 3. The system ofclaim 2 wherein said means for collecting object content includes:meansfor identifying all externally accessible object data and object methodsin said specified object; means for determining an addressable locationwithin said executable object specification for each of said externallyaccessible object data and object methods; and means for storing saididentified object data and object methods and said addressable locationsin said externally accessible data structure.
 4. The system of claim 3wherein said means for storing further comprises means for ordering saididentified object data and object methods and addressable locations inaccordance with an order specified by said means for specifying.
 5. Thesystem of claim 1, further comprising:client application means foraddressing said object content using said defined binary interface. 6.The system of claim 5, further comprising:means for respecifying objectcontent without modifying said client application means.
 7. A computerimplemented method of creating a dynamic binary interface to acollection of reusable computer implemented processes, the methodcomprising the steps of:receiving from an input device object interfacedefinition specification for said reusable computer implementedprocesses; transforming said object interface definition specificationsinto a format usable by any one of a plurality of computer processlanguages; starting execution of a client process referencing saidcollection of reusable processes; accessing said plurality of reusablecomputer implemented processes and constructing an externallyaddressable binary interface for said client process.
 8. The method ofclaim 7, wherein the step of receiving includes receiving aspecification identifying a subset of features of each of said pluralityof computer implemented processes as externally accessible, and whereinthe step of dynamically accessing said plurality of reusable computerimplemented processes comprises the steps of:determining the externallyaccessible features of said computer implemented processes according tothe order specified; and constructed a data structure containing each ofsaid externally addressable features and a location identifier for saidfeature within said computer implemented process.
 9. The method of claim8, wherein said externally addressable features are object methods. 10.The method of claim 8, wherein said externally addressable features areobject data.
 11. The method of claim 8, wherein said externallyaddressable features include both object methods and object data.